How to Fix Pluggable.php File Errors in WordPress

WordPress allows website owners and plugins to overwrite or override core functions. These functions can be located within the pluggable.php file. If a WordPress plugin or some custom code snippet you’ve added in incorrectly handles one of the core functions, then you’re likely to see an error that reads something like:

Warning: Cannot modify header information – headers already sent by (output started at /home/username/demosite/wp-content/themes/mytheme/functions.php:1035) in /home/username/demosite/wp-includes/pluggable.php on line 1179

Sometimes, you might be able to work on your site with this error displaying within the admin error but causing no problems. However, it is not advisable. And it is one of the easiest WordPress errors you can fix.

It is never a great idea to edit the core WordPress file. Even if there is an error within it, you should likely find what is causing the issue, like the plugin or snippet code. This is where the error is really is and not within the core programming file. If you fix the plugin/snippet code, you’ll fix the website.

So, the first thing you should do is to look at the location mentioned within the error messages.

For example in the code mentioned above, you can see that the error is located within the functions.php file on line 1035.

Therefore, you need to open your FTP software, access your website’s files and find the functions.php file. Within there, find the line and see if you can see the error. In most cases it is an extra space or typo. If you remove that, the error will be fixed.

If you’re struggling to know where the problem is, try uninstalling the plugins/themes you’ve recently installed. Do this one-by-one starting with the latest to update or be installed and work backwards. If the message goes after a theme/plugin has been deactivated, then that was the problem plugin/theme.

You can then find a replacement theme/plugin to find the same functionality.